Photo-luminescence is a spectral property of materials and is defined as the emission of light on exposure to invisible radiation or light of a different wavelength. Light energy which is absorbed by a crystal is normally lost in the internal vibrations of the crystal lattice. Luminescent materials which store the light energy and then reemit light at a lower energy are called phosphors. Most phosphors are excited with high energy photons in the UV region of the electromagnetic spectrum or by electrons from X-ray sources. The distinction between a fluorescent and phosphorescent material or phosphor is based on the time scale of the emission process. Materials which emit light on the scale of less than 1 microsecond are termed fluorescent. Phosphors emit light after the excitation source is turned off. This light may continue from tens of microseconds to several minutes.
Phosphors are used in a variety of applications, such as fluorescent lights, cathode ray and television screens, computer display devices, x-ray intensifying screens, brighteners in textiles and paper, and in phosphorescent analysis equipment.
Most phosphors are inorganic solids prepared from a metal cation, a nonmetal anion and an activator. The activator is usually a transition or a rare earth element which when added in small amounts (0.1% to 5%) can alter the color of the emitted light. For example, zinc sulfide (ZnS) can be activated with copper to give a blue-green emitting phosphor with a short lifetime of 35 microseconds. If copper is replaced with silver in the zinc sulfide lattice the emitted color is shifted to blue with a similar lifetime. Addition of the element cadmium (Cd) to ZnS:Ag increases the wavelength to green and extends the lifetime to 200 microseconds. The inorganic phosphor ZnCdS.sub.2 :Ag can actually be tuned in color by varying the Cd level from blue (no Cd) to red (high Cd).
Unfortunately, most of the phosphors currently in use are toxic and cannot be used in many applications. A need exists for phosphors and phosphor containing compositions that are non-toxic or which do not have the other disadvantages of the phosphors that are currently available for use.